Ever since manufacturing a continuous glass sheet over a layer of tin was in 1959 (float process) was achieved, applications thereof have diversified encompassing many markets; from applications in the construction market to its use with complex forms in the automotive market.
One of the applications of float glass is the protection of works of art and portraits, along with a frame made of wood or other materials. The primary purpose of covering artwork is the physical protection of the same from factors such as humidity, heat and stains. Glass is also used because it provides good levels of protection from UV rays which can attack the pigments of artwork whose protection is intended and eventually cause discoloration.
However, a common problem that occurs when using flat glass as protection for works of art is that when a viewer stands before a piece framed with this material, instead of seeing the work of art, the viewer will see its own reflection. In order to eliminate this effect caused by light reflection, several alternatives have been researched and the most widespread solution is the use of diffuse flat glass. Now, the way in which diffuse flat glass avoids reflections is by treating its surface with acidic solutions. This prevents glare by adding irregularities to the surface.
Generally, in making this diffuse glass, clear flat glass is used due to its high market availability and low cost compared to other glass types. Typical thickness of this glass ranges from 2.0 mm to 2.5 mm. Conventional clear glass with this thickness has a light transmittance of about 90% and a reflection of 8%. By giving glass an acid treatment, the reflection level is lowered to about 1%.
There is another raw materials option with low contents of iron that has been used in recent years but has a higher cost. The advantages of using this type of raw material is that it has a higher light transmission, around 92%, which allows a better perception of the image that is being protected. By performing the same acid treatment, it is possible to obtain the same level of reflection that is achieved with conventional clear glass which is 1%.
There are other options that can be used for the same purpose, although they involve the use of a complex or high-tech process, thereby making their manufacturing cost much higher. One of these options is anti-reflective (AR) glass manufactured by cathode pulverizing technology (sputtering).
Because of its cost, this type of glass is normally used for high value applications such as monitor screens or windows that are part of display cases. The most common configurations used in this type of products consists of alternating one high refractive index film (e.g. TiO2) with one of low refractive index (e.g. SiO2). Generally, the level of reflection achieved depends on the number of alternating films, where the product having lower performance has at least 4 films.
There are other materials that have been used trying to obtain a behavior similar to that of glass but showing a lower light reflection, one of them is acrylic sheet, which has a high light transmission and an optical quality similar to that of glass. It shows a great advantage against glass from a density viewpoint, since acrylic sheet is a very light material, but it has the disadvantage of being easily scratched and retains static charge which is detrimental to certain types of art (technique of pastel and charcoal), so it has not been very successful for anti-reflective applications.
Moreover, since the 40s attempts have been made to alter the surface of glass and give it a diffuse appearance by etching. For example, U.S. Pat. No. 2,348,704 (1944) issued to Frederick W. Adams proposes reducing reflection of light coming from a glass surface by using acids. This forms a thin film which is an interference between the exposed film surface and the glass surface covered by the film. The study described by Adams consists in treating glass with a strong mineral acid, in this case nitric acid 0.5 N in order to remove the basic glass components that are on the surface without attacking the SiO2. Subsequently, a dilute solution of hydrofluoric acid 0.1% by volume is used. It is well known nowadays that the use hydrofluoric acid in such low concentrations entails a long attack time to obtain the anti-reflective characteristics obtained by Adams, therefore it is not very practical to apply these findings in a production process.
U.S. Pat. No. 2,486,431 issued to Frederick H. Nicoll and Ferd E. Williams also describes the use of a solution of 0.5N nitric acid. Flat glass is immersed in this solution to generate a degradation of the glass surface microscopically eliminating planarity of the surface and forming porosity therein. Once this is achieved, the reaction with nitric acid is interrupted to continue flat glass immersion in a solution of hexafluorosilicic acid with concentrations ranging from 0.3 to 3.0 moles/liter working at temperatures between 35 and 55° C. Like the work done by Adams, again the problem of working with very low concentration solutions which need to be changed very quickly in a production process is presented. This is because each sheet immersed in the Hexafluorosilicic acid solution weakens the acid concentration, therefore in each batch that is made to obtain diffuse flat glass, adjustments are necessary to immersion times making it an uneconomic and unstable process.
In U.S. Pat. No. 2,461,840 issued to Frederick H. Nicoll, another method was explored to obtain diffuse flat glass, whereby it was not necessary to immerse the flat glass sheet in acid. Nicoll found that hydrofluoric acid vapor can generate a diffuse surface in the flat glass with no need to previously use other mineral acids. To accomplish this purpose, a tank with a wax cover was designed where a 1% solution of hydrofluoric acid was placed. This tank in turn was immersed in a larger tank which contained water, which was used to control the temperature of hydrofluoric acid present in the main tank. Subsequently, a flat glass sheet was placed over the main tank, which would be exposed to attack from hydrofluoric acid vapor, generating a diffuse surface on one of the sheet faces. By using this set of tanks, the vapor pressure of hydrofluoric acid can be manipulated to decrease attack times on the flat glass sheet. The main disadvantage of this process is that it can be applied only to one of the two flat glass surfaces, in addition to requiring long times to obtain the desired finish (minimum time 40 minutes), whereby scaling this type of batch process to industrial level does not seem to be profitable.
There are other methods not using acids to reduce reflection of light from the surface of flat glass, as described by Cook et al. (U.S. Pat. No. 4,434,191) where the possibility of using electrolyte solutions having a dissociation constant greater than 10-6 at a temperature of 20° C. is suggested. This type of method permits treating flat glass by immersion in electrolyte solutions, which have a neutral pH close to 7.0. In particular, by using this method it is possible to generate a high quality anti-reflective coating on the surface of flat glass which can be used for optical applications. The disadvantage of this method, like those mentioned above, is that it requires very long exposure times of flat glass to the electrolyte solution. Cook mentions in his work that normal times to achieve the anti-reflective coating range from 12-90 hours. Unfortunately, these times prevent having an industrial production unless having extremely large containers, which would involve a very high investment.
Efforts have made to obtain a production process for treating both surfaces of glass simultaneously, but undoubtedly the processes closer to achieving the desired goal are those involving immersion of flat glass sheets in acidic solutions.
Per the above, the present application discloses a batch type industrial process to produce several diffuse finish sheets of flat glass simultaneously, reducing considerably the value of reflection of specular light allowing applying the obtained product for protection of photographs and artwork.